Method of plating semiconductor wafer and plated semiconductor wafer

ABSTRACT

The present invention provides a plating apparatus and a method of plating, which improve the uniformity of the plate coat thickness without changing the flow velocity of feeding the plating solution.  
     By providing an aperture  14   a  at the center of a meshed anode electrode  14  of a plating apparatus thereby to obtain such an electric field density distribution, between the meshed anode electrode  14  and a wafer  101,  that is lower in the central portion of the wafer than in the portion along the edge.

BACKGROUND OF THE INVENTION

1. 1. Field of the Invention

2. The present invention relates to a plating apparatus and a plating method for forming a uniform plating layer on a semiconductor substrate.

3. 2. Description of the Prior Art

4.FIG. 7A shows a face-up type plating apparatus of the prior art wherein surface of a wafer 101 is plated while being arranged to face upward, and FIG. 7B is an enlarged view of a sealing portion of the wafer 101. In the drawing, the numeral 1 denotes a wafer processing vessel, 2 denotes a plating solution nozzle, 2 a denotes holes of a drain-board, 3 denotes a plating solution supply pipe, 4 denotes a plating solution discharge pipe, 5 denotes a drain pipe, 6 denotes a plating tank, 7 denotes a plating solution, 8 denotes an upper portion of the wafer processing vessel, 9 denotes a lower portion of the wafer processing vessel, 10 denotes a cathode contact, 11 denotes a sealing material, 12 denotes a nitrogen gas injection release, 14 denotes a meshed electrode, 16 denotes an auxiliary sealing material and 101 denotes a wafer.

5. In the plating apparatus described above, the plating solution 7 supplied through the plating solution supply pipe 3 is discharged through the plating solution discharge pipe 4, and is circulated throughout the period of plating process. A specified voltage is applied across the meshed anode electrode 14 and the wafer 101 via the cathode contact 10, thereby to form a plating coat on the wafer 101 surface. In such a face-up type plating apparatus, because the wafer surface is arranged to face upward, deposition of air bubbles onto the wafer surface can be prevented and plating coat of better quality can be formed in comparison to the face-down plating method where the wafer is arranged to face downward.

6.FIG. 8 shows a distribution of a plate coat thickness over the surface of a 4″ wafer plated with Au in the above plating apparatus with a current density of 5 mA/cm² for a plating time of 12 minutes, where a distance from the wafer edge is plotted along the axis of abscissa and plate coat thickness is plotted along the axis of ordinate. As is clear from FIG. 8, the plate coat thickness shows a W-shaped distribution which has a peak at the center of the wafer and increases toward the edge.

7. Through an investigation into the cause of such a distribution of the plate coat thickness, it was found that the distribution of the coat thickness is greatly affected by the distribution of the quantity of transported ions of the plating metal, which is determined by the flow velocity distribution of the plating solution, and the distribution of electric field in the wafer surface. Specifically, in the plating apparatus described above, because a flow velocity of the plating solution is highest, and accordingly the transported quantity of ions of the plating metal is largest, at the center of the wafer which is located just below the plating solution supply pipe 3, a plate coat is formed with the largest thickness at the center, while the electric field is concentrated at the edge which leads to the plate coat being formed with the second largest thickness along the edge of the wafer.

8. On the other hand, such a method may be used as the flow velocity of the plating solution 7 supplied through the plating solution supply pipe 3 is made slower, thereby reducing the distribution of the flow velocity of the plating solution in the wafer surface. However, when such a method is used, the plating solution 7 becomes stagnant locally on the wafer surface, resulting in lower quality of the plating.

SUMMARY OF THE INVENTION

9. Thus, an object of the present invention is to provide a plating apparatus and a method of plating, which improve the uniformity of the plate coat thickness without changing the flow velocity of feeding the plating solution.

10. The present inventors have intensively studied. As a result, it has been found that unevenness in the plate coat thickness due to the flow velocity distribution of the plating solution can be mitigated and uniform distribution of the plate coat thickness can be achieved over the wafer surface, by providing an aperture at the center of a meshed anode electrode of a plating apparatus thereby to obtain such an electric field density distribution, between the meshed anode electrode and the wafer, that is lower in the central portion of the wafer than in the portion along the edge, thus completing the present invention.

11. That is, the present invention provides an anode electrode installed to oppose a wafer, whereon a plating coat is to be deposited, for generating a specified electric field distribution over the wafer surface, which is a meshed electrode capable of supplying plating solution and has an aperture at the center of the meshed electrode.

12. Because the meshed anode electrode has the aperture at the center thereof, such an electric field density distribution that is lower in the central portion of the wafer than in the portion along the edge can be obtained by using the meshed electrode as the anode to form electric field between the electrode and the wafer.

13. Thus the flow velocity of the plating solution can be made lower in the central portion of the wafer than in the portion along the edge, making it possible to mitigate the unevenness in the plate coat thickness due to the flow velocity distribution of the plating solution which has been a problem for the prior art, thus improving the uniformity of plate coat thickness over the wafer surface.

14. The meshed anode electrode may either such an electrode made by weaving a thread-like material as shown in FIG. 2A, or an electrode made by punching holes through a sheet as shown in FIG. 2B.

15. The present invention also provides a plating apparatus which comprises, a plating tank wherein a wafer is placed so that the plating surface faces upward, plating solution supply means for causing the plating solution supplied from above onto the plating surface of the wafer at the center thereof to flow from the center of the plating surface of the wafer toward the periphery, and a meshed anode electrode installed to oppose a wafer for generating an electric field distribution by using the wafer as a cathode, wherein an aperture is made at the center of the meshed anode electrode to obtain such an electric field density distribution that is lower in the central portion of the wafer than in the portion along the edge.

16. Because the plating apparatus of the present invention uses the meshed anode electrode which has the aperture at the center thereof, such an electric field density distribution that is lower in the central portion of the wafer than in the portion along the edge can be obtained and, as a result, unevenness in the plate coat thickness due to the flow velocity distribution of the plating solution can be mitigated, that is, an increase in the plate coat thickness at the central portion of the wafer due to the flow velocity distribution of the plating solution can be suppressed by decreasing the electric field density in the central portion of the wafer, thereby making it possible to improve the uniformity of the plate coat thickness over the wafer surface.

17. The meshed anode electrode is a circular electrode having a diameter nearly equal the wafer diameter, and the aperture of the meshed anode electrode is preferably a circular aperture having a diameter of 40 to 80% of the wafer diameter.

18. This is because it is most preferable for improving the uniformity of the plate coat thickness to use the meshed anode electrode which has the same circular shape as the wafer and also has a circular aperture of diameter 40 to 80% of the wafer diameter.

19. The present invention also provides a method of plating the surface of a wafer, comprising the steps of causing the plating solution supplied onto the plated surface of the wafer to flow from the center of the plated surface of the wafer toward the periphery, generating an electric field between the wafer and the meshed anode electrode which is arranged to oppose the wafer, and generating such an electric field distribution that mitigates the unevenness in the plate coat thickness caused along the flow of plating solution by using the meshed anode electrode having the aperture at the central portion thereof.

20. By using such a method, a uniform plating coat can be obtained.

21. The present invention also provides a wafer for semiconductor devices provided with a plating layer plated by the above method, wherein the distribution of the plating layer thickness on the wafer is about 10%, more specially 5%.

BRIEF DESCRIPTION OF THE DRAWINGS

22.FIG. 1A is a sectional view of the plating apparatus according to the present invention.

23.FIG. 1B is a partially sectional view of the plating apparatus according to the present invention.

24.FIGS. 2A and 2B are plan views of the meshed anode electrode according to the present invention.

25.FIG. 3 shows a relation between the diameter of aperture made in the meshed anode electrode and the plate coat thickness uniformity when the plating apparatus of the present invention is used.

26.FIG. 4 shows a thickness distribution of the plating coat formed by using the plating apparatus of the present invention.

27.FIG. 5 is a sectional view of the process for producing semiconductor devices using the plating apparatus of the present invention.

28.FIG. 6 is a plane view of the semiconductor device provided with the plating layer plated by the method of the present invention.

29.FIG. 7A is a sectional view of the plating apparatus of the prior art.

30.FIG. 7B is a partially sectional view of the plating apparatus of the prior art.

31.FIG. 8 shows a thickness distribution of the plating coat formed by using the plating apparatus of the prior art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

32.FIG. 1 shows a plating apparatus according to one embodiment of the present invention, where the numerals which are identical with those of FIG. 6 denote identical or corresponding components.

33. According to the plating method of the present invention, first, an upper portion 8 of a wafer processing vessel and a lower portion 9 of the wafer processing vessel are separated, with a wafer 101 being placed at the lower portion 9 of the wafer processing vessel by robotic transfer equipment, for example, so that the plating surface faces upward, and the lower portion 9 of the wafer processing vessel moves upward or the upper portion 8 of the wafer processing vessel moves downward, in order to bring the wafer 101 and a cathode contact 10 (refer to FIG. 7B) included in a sealing material 11 into contact with each other, and joint between the upper portion 8 of the wafer processing vessel and the lower portion 9 of the wafer processing vessel is sealed by sealing materials 11, 16.

34. Then plating solution 7 is supplied from a plating solution supply pipe 3 which is installed above the center of the wafer 101 to fill the wafer processing vessel 1, while the plating solution 7 flows through the holes of a drain-board 2 a and the meshed anode electrode 14 onto the wafer 101 and flows from the center toward the periphery of the wafer 101, eventually to be discharged from a plating solution discharge pipe 4 located above the periphery and circulated.

35. As the plating solution 7, an Au plating solution including sodium gold sulfite or potassium gold cyanide as a major component is commonly used, while temperature of the plating solution is usually set to about 50 to 70° C.

36. When such a plating solution 7 is circulated, flow velocity of the plating solution 7 over the wafer surface becomes maximum at the center of the wafer 101 and decreases toward the periphery, in a concentric distribution. Therefore, ions of the plating metal which are transported are concentrated in the central portion of the wafer 101 and, when the plating apparatus of the prior art shown in FIG. 6 is used, the plating coat becomes thicker in the central portion of the wafer.

37. According to the present invention, on the other hand, a circular aperture 14 a as shown in FIG. 2A, for example, is installed at the center of the meshed anode electrode 14 thereby to generate such an electric field between the meshed anode electrode 14 and the wafer 101 as the electric field density is lower in the central portion of the wafer 101 (that is, electric lines of force are sparsely distributed), while leaving the flow velocity of the plating solution as it is.

38.FIG. 2A shows a meshed anode electrode having an aperture 14 a provided at the center of the woven meshed electrode of a Ti/Pt-plated sheet, which may be replaced by a meshed anode electrode made of a Pt/Ta/Pt-clad material having a plurality of punched holes and an aperture 14′a provided at the center of the electrode. Diameter of the meshed anode electrode is preferably about the same as the diameter of the wafer to be plated and, in this embodiment, the diameter of the meshed anode electrode is preferably about 120 mm because 4-inch wafer is assumed to be the object of the plating treatment.

39. The electric field distribution over the wafer may be controlled by a method, for example, disclosed in Japanese Utility Model Kokai No. 6-37354 wherein a baffle plate is installed between the meshed anode electrode 14 and the wafer 101, although such a method changes the direction of flow of the plating solution, which results in the uniformity of thickness distribution of the plating coat being deteriorated contrary to the intention.

40. Thus the electric field between the meshed anode electrode 14 and the wafer 101 has lower electric field density in the central portion of the wafer, so that the plating reaction in the central portion of the wafer 101 is suppressed, and an effect of thinning the plating coat is obtained in the central portion of the wafer 101.

41. As a result, such an electric field density distribution mitigates the unevenness in the plate coat thickness due to the flow velocity distribution of the plating solution, namely the increase in the plate coat thickness in the central portion of the wafer due to the flow velocity distribution of the plating solution can be suppressed by decreasing the electric field density in such a portion, making it possible to improve the uniformity of the plate coat thickness on the wafer surface.

42.FIG. 3 shows a relation between the aperture diameter (diameter of hole in the anode) and the measured value of the thickness uniformity (3 σm:m mean thickness at 21 points) when the aperture 14 a of the meshed anode electrode 14 of 123 mm in diameter is changed.

43. From FIG. 3, it can be seen that, while the coat thickness uniformity is 60% in case the diameter of the anode hole is zero, namely in the case of the conventional configuration where the aperture is not provided, the coat thickness uniformity is improved to 10% or lower by providing an aperture and, when the aperture diameter is 45 mm, in particular, an extremely good uniformity of the coat thickness of about 5% can be obtained.

44. The results shown in FIG. 3 and others show that the aperture of the meshed anode electrode is preferably formed in the central portion of the electrode with a diameter in a range of about 40 to 80% of the meshed anode electrode diameter which is made nearly equal to the wafer diameter.

45.FIG. 4 shows a distribution of a plate coat thickness over the surface of a 4″ wafer which is plated by using the meshed anode electrode 14 that is made so that diameter of the anode aperture is 75% of the anode diameter, where a distance from the wafer edge is plotted along the axis of abscissa and plate coat thickness is plotted along the axis of ordinate. Plating conditions are the same as those of the case shown in FIG. 8: current density is 5 mA/cm² and plating time is 12 minutes.

46. By comparing the distribution of the plate coat thickness obtained by using the meshed anode electrode 14 according to the present invention and the distribution of the plate coat thickness obtained by using the conventional meshed anode electrode shown in FIG. 8, it is clearly seen that the coat thickness at the center is reduced to about 4.5 μm which is near the mean plating thickness in the case of FIG. 4 in contrast to 6 μm in the case of FIG. 8, and the decrease in coat thickness at a position 15 mm inward from the edge of wafer observed in FIG. 8 is eliminated in the case of FIG. 4.

47. This is presumably because electric field density is lower (electric lines of force are sparsely distributed) in the vicinity of the center of the wafer in case the meshed anode electrode of the present invention is used, plating reaction is suppressed in the central portion of the wafer compared to the prior art. Also because the plating reaction is suppressed in the central portion, ions of the plating metal which would be consumed in the plating reaction in the central portion in the prior art are carried by the flow of plating solution and supplied along the periphery of the wafer, and therefore the decrease in the coat thickness in a portion about 15 mm inward from the edge of wafer which is observed in FIG. 8 is supposedly suppressed by the supply of ions of the plating metal.

48.FIG. 5 shows a method of producing semiconductor devices which uses the plating apparatus provided with the meshed anode electrode described above.

49. First, a plating feeding layer (a laminated layers of Ti/Au, TiW/Au, Cr/Au, etc., for example) 102 is formed on the semiconductor wafer 101 made of Si, GaAs or the like as shown in FIG. 5A by vapor deposition or sputtering deposition.

50. Then a photoresist pattern 103 is formed by image transfer on the semiconductor wafer 101 whereon the plating feeding layer 102 has been formed as shown in FIG. 5B. At this time, a contact pattern 103 a where the photoresist pattern 103 is not formed is left at one or more places along the edge of the wafer 101.

51. Then the wafer 101 is placed on the lower portion 9 of the wafer processing vessel of the plating apparatus (refer to FIG. 1A) of the present invention, while placing the upper portion 8 of the wafer processing vessel having a sealing material 11 such as O-ring over the wafer 101 and sealing around the wafer with the sealing material 11. At this time, the cathode contact 10 incorporated in the sealing material 11 and the contact pattern 103(a) provided around the semiconductor wafer 101 are electrically connected to each other (refer to FIG. 1B).

52. Now the plating solution 7 is introduced into the wafer processing vessel 1 through the plating solution supply pipe 3. The plating solution 7 is supplied from the plating solution supply pipe 3 located above the center of the wafer 101 through holes 2 a of the drain board and the meshed anode electrode 14 onto the wafer 101, then flows over the surface of the wafer 101 from the central portion thereof toward the periphery, and is discharged from the discharge pipe 4 installed above the wafer edge to the outside of the wafer processing vessel 1, thereby to circulate.

53. Then an electric field with current density of several milli-Amperes/cm² to several tens of milli-Amperes/cm² is applied across the meshed anode electrode 14 and the cathode, namely the feeding layer 102 on the wafer 101, in constant-current electrolysis, for example, thereby to carry out plating on the surface of the wafer 101 by using the photoresist pattern 103 provided on the wafer 101 as the mask.

54. In the plating apparatus (FIG. 1A) described above, because the electrode having a hole at the center thereof as shown in FIGS. 2A and 2B is used as the meshed anode electrode 14, electric field density is lower in the central portion of the wafer 101 than in the periphery, thus it is made possible to form the plating coat 104 having more uniform thickness than that obtained by using the conventional plating apparatus (FIG. 6).

55. Last, the wafer 101 is taken out of the wafer processing vessel as shown in FIG. 5D and, after the resist pattern 103 is removed by means of an organic solvent treatment, oxygen ashing or the like, the plating feeding layer 102 is removed, from portions where the plating coat 104 is not formed, by RIE or ion milling process, thereby to obtain the desired plating pattern.

56. The above producing method can be applied to the formation of Au bumps on Si wafer, GaAs wafer or the like, Au plating of wiring and plating of electrode.

57.FIG. 6 shows the plane view of the semiconductor device with the plating layer plated by the method of the present invention. In the drawing, 21 denotes a plating layer.

58. Generally, plurality of the semiconductor devices are made of one wafer. Using the method of the present invention, good uniformity of the plating layer (the distribution of the plating layer thickness) of about 10%, more specially 5%, in the wafer can be obtained, so that good uniformity of the plating layer between the semiconductor devices made of the wafer can be obtained. 

What is claimed is:
 1. A plating apparatus comprising: a plating tank wherein a wafer is placed so that the plating surface faces upward, plating solution supply means for causing plating solution supplied onto the plating surface of the wafer at the center thereof to flow from the center of the plating surface of the wafer toward the periphery, and an anode electrode installed to oppose the wafer for generating an electric field distribution by using the wafer as a cathode, wherein an aperture is made at the center of the anode electrode to obtain such an electric field density distribution that is lower in the central portion of the wafer than in the portion along the edge.
 2. The plating apparatus according to claim 1 , wherein the anode electrode is a circular electrode having a diameter nearly equal to the wafer diameter.
 3. The plating apparatus according to claim 1 , wherein the aperture of the anode electrode is a circular aperture having a diameter of 40 to 80% of the wafer diameter.
 4. The plating apparatus according to claim 1 , wherein the anode electrode is a meshed electrode through which the plating solution can be supplied.
 5. The plating apparatus according to claim 4 , wherein the meshed electrode is coated by Ti/Pt plated layers.
 6. The plating apparatus according to claim 1 , wherein the anode electrode is a board electrode with a plurality of holes through which the plating solution can be supplied.
 7. The plating apparatus according to claim 6 , wherein the board electrode is made from Ta board cladded by Pt layers.
 8. A method of plating a surface of a wafer, comprising the steps of: causing plating solution supplied onto the plating surface of the wafer to flow from the center of the plating surface of the wafer toward the periphery, generating an electric field between the wafer and an anode electrode which is arranged to oppose the wafer, and generating such an electric field distribution that mitigates the ununiformity in the plate coat thickness caused along the flow of plating solution by using the anode electrode having the aperture at the central portion thereof.
 9. The method according to claim 8 , wherein the anode electrode is a meshed electrode through which the plating solution can be supplied.
 10. The method according to claim 8 , wherein the anode electrode is a board electrode with a plurality of holes through which the plating solution can be supplied.
 11. The wafer for semiconductor devices provided with a plating layer plated by the method of claim 8 , wherein the distribution of the plating layer thickness on the wafer is about 10%.
 12. The wafer for semiconductor devices provided with the plating layer plated by the method of claim 8 , wherein the distribution of the plating layer thickness on the wafer is about 5%. 